The morphologic determinants of mitral systolic anterior motion (SAM) and subaortic obstruction in hypertrophic cardiomyopathy (HCM) are not completely understood, particularly the contribution of mitral valvular size. To clarify this issue, in the present study mitral valve specimens from 43 patients with HCM and basal outflow obstruction (produced by systolic contact between mitral leaflet and ventricular septum) were utilized to relate morphometric measurements of leaflet area to certain morphologic and functional assessments of left ventricular outflow tract geometry and valvular motion obtained from echocardiograms in the same patients. Twenty-four patients (56%) had mitral valves of normal size (leaflet area less than 12.0 square cm) and 19 patients (44%) had enlarged and elongated valves (area equal to or greater than 12.0 square cm). Compared to normal sized mitral valves, the enlarged valves were situated more posteriorly in a larger left ventricular outflow tract and also had greater systolic excursion of anterior leaflet with a distinctive sharp angled bend and localized contact lf leaflet tip with ventricular septum. In contrast, normal sized mitral valves often showed a different mechanism of outflow obstruction with more restricted SAM (with little or no bending) and septal contact usually achieved by greater portions of the anterior leaflet and contiguous chordae. In both normal sized and enlarged mitral valves, the pattern of SAM reflected the distribution of fibrous thickening. An angled bend of anterior mitral leaflet occurred only if the central and distal portions of the leaflet were relatively free of fibrous thickening while a more restricted motion pattern was characteristic of fibrous thickening involving virtually the entire leaflet.